In general, storage device is mainly constituted by a control unit and a data storage medium (for example, a flash memory). The data storage medium includes a plurality of data blocks, and each data block includes a plurality of data pages. The control unit is electrically coupled to the data storage medium and configured to perform data access or data erase on the data pages in the data blocks. It is understood that weakened data retention capability may result in data integrity issue; therefore, the control unit may use mechanisms, such as checking whether the bit error count (BEC) of the data pages in the data blocks is greater than a threshold, to determine whether the data stored in the data pages in the data blocks can be accessed normally. Once it is determined that the bit error count of the data pages in the data blocks is greater than the threshold, the control unit performs a data swap operation to move the stored data to another data block so as to ensure data integrity. However, the reasons to an increased bit error count may vary and frequent data moving may result in poor system performance. Therefore, the present invention provides a technical solution to the aforementioned problems.